Horseweed (Conyza canadensis) is a dicot plant in the asteraceae family. A single amino acid substitution from Tryptophan 574 to Leucine has led to resistance to ALS inhibitors as indicated in the table below.

Horseweed

Chemical Family

Example Herbicide

Resistance Level

Imidazolinones

Imazethapyr

Resistant > 10 fold

Pyrimidinylthiobenzoates

Bispyribac-Na

Resistant > 10 fold

Sulfonylureas

Chlorsulfuron

Resistant > 10 fold

Triazolopyrimidines

Chloransulam-methyl

Not Determined

Sulfonylaminocarbonyltriazolinone

Flucarbazone-Na

Not Determined

NOTE

Authors suggest resistance to pyrithiobac-sodium (a PTB) may be due to something other than, or in addition to, the ALS mutation.

Conyza canadensis (CC) and Conyza bonariensis (CB) are troublesome weeds around the world. Extensive use of herbicides has led to the evolution of numerous Conyza spp. herbicide-resistant populations. Seeds of 91 CC and CB populations were collected across Israel. They were mostly found (86 %) in roadsides and urban habitats, two disturbed habitats that had been dramatically impacted by human activities, thus we classify these species as anthropogenic. Although pyrithiobac-sodium was only used in cotton fields, 90 % of Conyza spp. populations were identified as pyrithiobac-sodium resistant, suggesting possible natural resistance to pyrithiobac-sodium. CC21 and CC17 C. canadensis populations were highly resistant to all tested ALS inhibitors due to a substitution in the ALS gene from Trp574 to Leu. They were also atrazine resistant due to a substitution in the psbA gene from Ser264 to Gly. The high level of imazapyr and pyrithiobac-sodium resistance observed in the CC10 population was due to an Ala205 to Val substitution. However, high resistance to sulfometuron methyl and pyrithiobac-sodium in population CC6 was due to a point mutation at Pro197 to Ser. All resistant plants of CC21 population showed both psbA (Ser264 to Gly) and ALS (Trp574 to Leu) substitutions, leading us to the conclusion that the attempt to overcome resistance to one mode of action by overuse of another will most likely lead to multiple herbicide resistance. Furthermore, we concluded that only individuals that carry both mutations could survive the shift between the two modes of action and overcome the fitness cost associated with the PSII resistance..